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Getting set to build a laser pointer here

on April 13, 2015

Depending on how this goes I might post the project here. As of yet I have not built anything though. Although I have designed some simple circuits already. I'm waiting for the parts I ordered to arrive. I have researched the topic a bit on the 'net and I am thinking about building a very simple current regulator for the laser doide I am getting.

This is all new to me though so I figured I would post my schematic here to see what kind of feedback I get about it. I may have missed something critical that I am not aware of.

on April 14, 2015

pfred,

Your schematic is looking good. Glad to see you did not put a capacitor on the output since you are using it for a constant current regulator. Some people say to put them on there to stabilize things, but you should not have one when using it as a current regulator.

I can't wait to see your project when it is done.

What voltage is your laser diodes you are getting? If you have time to wait...here are some that I bought..but they are in china and will take a little while to arrive....

on April 14, 2015

Your schematic looks fine, and nearly matches the sample circuit from the 317 datasheet. Using your values, it looks like you would be regulating to a 4 mA current. Your results will vary depending on the load you are using. No load should result in a 1.2V output, a diode may result in the forward voltage of the diode being the output, but you may need a parallel resistive load to trigger the 317 to get the output voltage above 1.2V. When you get your parts, breadboard and test it out.

Since you said you were building a laser pointer, you may want to look into a voltage thief circuit instead so you can run your pointer from a since cell battery, instead of what I'm guessing would be a 9V. It does need a few parts that you may not have, but overall it's not a complex circuit either.

on April 15, 2015

sgray74 said:

pfred,

Your schematic is looking good. Glad to see you did not put a capacitor on the output since you are using it for a constant current regulator. Some people say to put them on there to stabilize things, but you should not have one when using it as a current regulator.

I can't wait to see your project when it is done.

What voltage is your laser diodes you are getting? If you have time to wait...here are some that I bought..but they are in china and will take a little while to arrive....

The Operating Voltage is 3V and the Operating Current is < 45 mA....just a thought.

Thanks for the reply. I was wondering about the output stability capacitor. I've heard not to use it, so I left it out. I am shooting for 5ma with my output. That is what the diodes I am getting are rated for. They are supposed to be 3V diodes too. I am getting a 10 pack, so I probably will blow a couple of them up, just to see what they can really do. It is not like I need 10 pointers. Just one good one should suffice.

I am thinking about heatsinking it too. They come in little brass holders, but I am thinking a larger aluminum heatsink might help them out? Until I actually get the things I don't really know about any of that. Supposedly this brass barrel they're in has an adjustable collimating lens in it. Some say the whole affair is very fragile too, so I might not be able to mount them into a press fit heatsink too easily without breaking them?

Then again I have some machine tools so I might be able to rig something up here.

on April 15, 2015

rtecoder said:

Your schematic looks fine, and nearly matches the sample circuit from the 317 datasheet. Using your values, it looks like you would be regulating to a 4 mA current. Your results will vary depending on the load you are using. No load should result in a 1.2V output, a diode may result in the forward voltage of the diode being the output, but you may need a parallel resistive load to trigger the 317 to get the output voltage above 1.2V. When you get your parts, breadboard and test it out.

Since you said you were building a laser pointer, you may want to look into a voltage thief circuit instead so you can run your pointer from a since cell battery, instead of what I'm guessing would be a 9V. It does need a few parts that you may not have, but overall it's not a complex circuit either.

For sure I am going to spend some time on a breadboard testing out various things. Ideally I'd like to run on these 7.2V rechargable tool batteries I have. To do that I might need to put a voltage regulator on those though. That shouldn't be an issue. I am getting 10 LM317 ICs with my diode order.

on April 16, 2015

pfred1 said:

For sure I am going to spend some time on a breadboard testing out various things. Ideally I'd like to run on these 7.2V rechargable tool batteries I have. To do that I might need to put a voltage regulator on those though. That shouldn't be an issue. I am getting 10 LM317 ICs with my diode order.

A 3V diode won't need voltage regulation, it just needs at least 3V. Like all diodes the laser diode will simply drop the voltage by its Vf and pass the rest through. So you'd have ~4.2V left over to burn off and 1.2V of that is handled by the current regulating circuit. The rest can go straight to ground (similar to using a zener as a shunt regulator). That assumes that the 317 doesn't see the diode as an open circuit since no power will flow until 3V is reached.

If on the other hand you used the 317 as a voltage regulator to say 3.5V and used a series resistor to burn off the excess 0.5V you would get a constant current setup yet again. You would want smoothing caps in that setup though.